The long-term objectives of this project are to characterize the interactions between th tubuloglomerular feedback (TGF) and myogenic mechanisms in the mediation of renal autoregulatory behavior and to delineate the roles of major paracrine systems in regulating afferent and efferent arteriolar resistance as well as differences in cortical and medullary microvascular dynamics. A major focus is on the cortical role exerted by the macula densa in the regulation of the renal microvasculature. Recent studies have indicates that multiple vasoactive agents are produced and released by macula densa cells. In this regard, extracellular ATP has emerged as an important regulator of afferent arteriolar tone and the major mediator of TGF responses. Particular novel issues are the potential roles of cyclooxygenase-2 (COX-2) derived metabolites and of endogenous carbon monoxide (CO) both directly and through their interactions with NO. For the next period of support, we will focus on the following: 1) the hypothesis that renal interstitial ATP, derived from macula densa cells, serves as the major paracrine agent mediating autoregulatory responses and TGF signals to regulate afferent arteriolar resistance, 2) the hypothesis that reduced salt intake augments the action of nNOS derived NO and COX-2 derived prostanoids to modulate the sensitivity of the TGF mechanism, 3) the interactions between the L-arginine-NOS-NO and the heme-hemeoxygenase-CO systems in the regulation of afferent and efferent arteriolar tone, and 4) the interactions among NO, ANG II and bradykinin as determinants of the differential sensitivity between cortical and medullary blood flow during reduced sodium intake. In vivo experiment sin dogs and rats will evaluate whole kidney function using standard techniques as well as single fiber needle laser-Soppler flowmetry for regional blood flow measurements, NO electrodes to determine interstitial NO activity, and microdialysis probes to assess interstitial fluid ATP concentrations In vivo studies in rats will also assess single nephron function and activity of the TGF mechanism. In vitro studies in rats and mice will provide assessment of single vessel diameter and blood flow responses in individual afferent and efferent arterioles. When appropriate, gene targeted mice that have reduced or enhanced expression of intrarenal paracrine factors or receptors will be utilized. The results will provide an improved understanding of how these intrarenal paracrine factors interact to produce a coordinate regulation of renal microvascular hemodynamics.